why do we need complex bifocals?
- when the prescription is the same in each eye, when a patient looks away from the optical centre we know they will be experiencing prism
- but because they are experiencing the same amount of prism in the same direction in each eye , it doesn’t create a problem
- however when a patient has anisometropia, when the patient looks away from the optical centre , they will experience a different amount of prism
what is anisometropia?
- unequal refractive power between the eyes
what does anisometropia result in?
- results in asthenopia and diplopia due to differential prism and aniseikonia
what is differential prismatic effect?
- different amounts of prism experienced by each eye when looking away from the OC, tolerance- only 2 prism dioptres vertically, more horizontally
how much does a px look below when they are reading?
- they look 8-10mm below to read
which patient can get used to differential prismatic effect ?
- patients that had anisometropia from a young age and worn there glasses and have a reasonable VA
which patinets can’t get used to differential prismatic effect ?
- when anisometropia happened suddenly due to pathology or not doing a cataract surgery
what is an aniseikonia ?
- aniseikonia - a difference in the retinal image size between the eyes resulting from the difference in prescription
- makes it difficult for the brain to fuse two images into one image because the image is falling into different retinal locations within each eye
- difference in image sizes is 1% of every dioptres anisometropia
- we can only tolerate 1%
- 1 dioptre= 1% difference in retinal image size
what can anisokonia cause?
- it can cause distortion , diplopia and can alter patient depth perception
what can we do to solve anisokonia ?
- we need to equal out the image size
- we can either make the larger image smaller or smaller image larger
how to make smaller image larger - to increase spectacle magnification?
-use seikonic/size lenses to increase magnification
what can we use to determine the prismatic effect?
- we can determine the prismatic effect at a point using Prentice’s rule
- P=CF
when is the near vision point ?
- near vision point: 8-10 mm down and 2mm in
what are the bifocal solutions?
- slab-off lenses/bi-prism
- removes unwanted base down prism from the bottom of the back of the lens
- this results in a line across the lens so an E or D seg with the seg top coinciding with the line looks best
why is it not a good idea to slab off a varifocal ?
- we can also slab off varifocal but not good from a cosmetic view
- good to add an anti-reflection coating which reduces the appearance of the line
what is a slab on ?
by adding prism to the bottom of the lens
what is the minimum prism we can remove in traditional slab off lenses?
- minimum of 2 prism dioptres base down unless freeform
which lens do we usually slab off?
- slab off the most negative
what is another bifocal solution ?
- franklin split
. DV and NV portions are completely separate lenses
. the OCs can be placed where required and prism can be surfaced into either portion to eliminate unwanted prismatic effect
. not cosmetically good
what is a prism controlled bifocal?
- this is another bifocal solution
- usually available in glass only , with visible round 30mm bifocal segment.
- up to 6 △ of extra prism is worked in any direction in the segment only
- always worked into back surface of lens so not visible from front
how to work out the base direction of what’s been put into the segment in prism controlled bifocal ?
- when looking at a prism control bifocal, if you want to work out the base direction of what’s been put into the segment.
- its the place where there is least differentiation between the edge of the segment and the lens
- the prism has been dropped in and base direction is thickest point of the prism
how to use different size round segment as a solution for patients who are struggling with differential prismatic effect/
- the largest segment is dispensed to the most positive eye ( it induces more base down prism in the segment for reading, to eliminate the base up of the distance portion)
how to work out difference in segment sizes ?
20 x differential △ / add
what is a fresnel prism ?
- another bifocal solution
- normally used a temporary measure
- will reduce the patient’s VA by approx 2 lines
- increases chromatic dispersion
- may be used for prism in any direction and available from
1 △ to 30 △ - frensel is a thin sheet of plastic which we can stick onto back surface of lens with water backing
- fresenl is made of lot prisms- thicker end of ridges represents the base direction
